Increased emphasis on environmental considerations is a driving force advancing the utilization of energy-curable ink technology in the printing and graphic arts industry. Energy-curable inks comprise ultraviolet light (UV-curable) and electron beam radiation (EB-curable) inks. Achieving good quality UV inks is strongly correlated with the final rheology of the system. Rheology is determined by the conformation that the vehicle adopts when wetting the pigment surface. Stable dispersion of the pigment in ink formulations, good ink flow and transfer on press, and subsequent color strength all depend upon pigment wetting.
Pigment wetting can be described as a chemical interaction between the binder and the pigment that helps prevent the flocculation process and enhances the color of the printed ink. The rheological advantages of a well-wetted system are good flow characteristics, low overall ink viscosity, high gloss and proper color development. UV-curable oligomers are available commercially. However, inks based upon conventional oligomers disadvantageously require substantial amounts of various photoinitiators to ensure acceptable photo-cure. Traditional photoinitiators (e.g., benzophenone) can be toxic, expensive, malodorous, and may contribute to film color, which can limit their applicability, in general, and may render them unsuitable for use in white and light-colored inks.
The amount of external photoinitiator in ink formulations can be significantly reduced by using the acrylate oligomer technology described in U.S. Pat. Nos. 5,945,489 and 6,025,410 (both Ashland, Inc.). These patents disclose uncrosslinked resins prepared via the Michael addition reaction of β-dicarbonyl compounds with multifunctional acrylates. The invention disclosed here demonstrates the advantageous use of these uncrosslinked resins alone or modified by reaction with and/or blending with additional materials for grinding different color pigments. These additional materials include a variety of acrylic monomers and oligomers, primary, secondary and tertiary amines, acid functional materials, dispersing agents, pigment wetting aids and others to enhance pigment wetting characteristics. The resulting pigment dispersions can be let-down with appropriate oligomers based on the same Michael Addition resin technology to give UV-curable screen, flexographic, ink-jet and lithographic inks.
The invention detailed herein comprises a family of pigment dispersions for radiation-curable printing ink applications. These dispersions are based on multifunctional acrylate resins formed by the reaction of acrylate monomers and oligomers with β-keto esters (e.g., acetoacetates), β-diketones (e.g., 2,4-pentanedione), β-keto amides (e.g., acetoacetanilide, acetoacetamide), and/or other β-dicarbonyl or Michael “donor” compounds that can participate in the Michael addition reaction combined with a variety of organic and inorganic pigments. An essential novelty of these pigment dispersion vehicles is that the inks based on them will cure under standard UV-cure conditions with very little traditional photoinitiator as compared to commercial formulations which require the addition of substantial amounts of photoinitiator.
The multi-functional polyacrylate oligomers which are used as pigment grinding vehicles in the present invention have dual chemical functionality. That is, they have both acrylic functionality and a labile ketone group that is capable of dissociating, upon exposure to UV radiation, to initiate free radical polymerization of the oligomer.
Pigment wetting and dispersion properties can be modified by various means including the use of additional or supplementary acrylate materials, using various β-dicarbonyl Michael donor compounds, or by simply varying the stoichiometry of the reactants which comprise the oligomer. Inks based on these pigment dispersions may be cured via chemical means, thermally, or by exposure to UV or electron beam radiation. Other materials, both reactive (e.g., conventional polyacrylates or acrylated oligomers) and non-reactive (e.g., thermoplastic polyacrylates such as PMMA), may also be incorporated into dispersion formulations to enhance the pigment wetting characteristics of these resins. Such additives include, but are not limited to, various acrylic monomers and oligomers; primary, secondary, and tertiary amines; and acid-functional monomers and oligomers.
Dispersion systems comprised of traditional monomers and oligomers often have compatibility issues with some of the above additives, giving the formulations chemist fewer options. However, dispersions built from the novel photo-curable Michael Addition resins described herein can incorporate a nearly unlimited variety of additives due to the chemical/architectural control possible in their synthesis. Thus, many more options are available to the formulator who must address challenges specific to each printing application.